The present invention relates to apparatus for removing unwanted particles from a photoconductor of an electrographic apparatus which has a developed image thereon so that the unwanted particles are not transferred to a copy sheet with the developed image.
Images are formed in an electrographic apparatus by moving a photoconductor in the form of a drum or flexible web past a series of stations in the apparatus. As this occurs, the photoconductor is charged, exposed to form a latent charged image on the photoconductor, and the image is then developed by moving it past a development station where charged toner particles of developer material are attracted to the charged latent image to develop the image. The developed image is transferred to a receiver sheet, such as a sheet of paper in a transfer station. The receiver sheet is advanced through a fusing station where the toner particles are heated and fused to the sheet. Particles remaining on the photoconductor are removed at a cleaning station prior to again charging the photoconductor and repeating the process.
During operation of an electrographic apparatus some unwanted particles may be deposited onto the photoconductor before it reaches the transfer station, and these particles should be removed in order to avoid imperfections in the image on the copy sheet. The unwanted particles may comprise, for example, toner aggregates or agglomerations (sometimes referred to as toner flakes), particles of carrier from the developer material, paper dust, or fibers of brushes used for cleaning the photoconductor. Particularly objectionable are any such unwanted particles that are present within the image area on the photoconductor prior to the time the image reaches the transfer station where it is to be transferred to a copy sheet or other receiver sheet.
The unwanted particles may be large in comparison to the small individual toner particles which form the developed image, and are sometimes referred to as "tent poles". When the receiver sheet and the photoconductor are brought into contact or close proximity for transfer of the image, the receiver sheet in the area around a large unwanted particle or tent pole is held away from the photoconductor by the particle. As a result, some of the small toner particles in the image area around the large particle on the photoconductor do not transfer to the receiver sheet. The effect on the final copy or transfer sheet is an area of low density toner image, sometimes surrounding a black spot when the unwanted particle also transfers to the receiver sheet. These defects in image quality should be avoided, especially in half tone and solid density areas of an image where image quality voids are most noticeable.
The developer material used for developing the latent images on the photoconductor sometimes comprises a two-component developer including ferromagnetic carrier particles and toner particles. The carrier particles should not be transferred to the photoconductor, but some do and they too will degrade the image if not removed before transfer to a copy sheet. These ferromagnetic carrier particles can be removed by a magnetic scavenger located downstream of the development area, as disclosed in commonly-assigned U.S. Pat. No. 3,543,720, which issued on Dec. 1, 1970 in the names of R. A. Drexler et al.
It also is known to use positive air pressure for removal of agglomerations or "tent poles" of material from a developed image on a photoconductor. A positive air pressure system and apparatus is disclosed in item 24942 found on pages 73 and 74 of the January, 1985 edition of Research Disclosure, published by Kenneth Mason Publications Limited, the old Harbourmaster's, 8 North Street, Emsworth, Hampshire P.O. 10 7DD, England.
Vacuum or negative air pressure also has been used to remove unwanted particles from a photoconductor. See for example, U.S. Pat. No. 4,014,065, issued Mar. 29, 1977 in the name of F. W. Hudson, which discloses a vacuum system for removing unwanted particles from the background area of a photoconductor. In accordance with the Hudson patent a uniform air flow across the photoconductor is provided by a chamber entrance port mush smaller than the exit port. Also, the vacuum apparatus is disclosed in connection with a photoconductor on the surface of a rigid drum where it is much easier to maintain a desired relationship between the entrance port and the photoconductor than with a flexible photoconductor.
When the photoconductor is in the form of a web, the photoconductor flexes and it is difficult to constantly maintain the desired relationship between the vacuum system and the photoconductor with the kind of accuracy required for removing unwanted particles from an image area of the photoconductor without also removing toner particles from the image area. More specifically, due to movement of the photoconductor relative to the vacuum system, the vacuum applied to the image area may not be great enough to remove the unwanted particles or may be so great as to remove not only the unwanted particles, but also the toner particles forming the image that is to be transferred to a copy sheet. Also, the vacuum system can contact the toner image on the photoconductor and damages the image, or contact and scratch the photoconductor unless the desired relationship between the vacuum system and photoconductor is maintained. Another problem that can occur if the proper relationship is not maintained between the photoconductor and a vacuum scavenger is that the vacuum system can tend to draw the photoconductor into the entrance slot of the vacuum system at the operating level of the vacuum system.
The difficulty in consistently maintaining a specific relationship between the vacuum system and a flexible photoconductor is even more difficult when a movable roller is urged against the photoconductor between the development station and the transfer station in order to tension the photoconductor. More specifically, the tension roller moves toward and away from the photoconductor in response to forces exerted on the photoconductor during operation of the electrographic apparatus, and this produces localized flexing of the photoconductor in the exact area where the vacuum system must be accurately spaced from the photoconductor in order to most effectively remove the unwanted particles without damaging a developed image or the photoconductor.